Some supporters had been waiting since the early hours of Monday morning to see the president.
A counter-demonstration against his appearance was also held nearby.
What did Mr Trump say?
During his roughly 80-minute speech, Mr Trump reiterated key themes of his winning 2016 campaign.
He pledged to continue a crackdown against illegal immigration, one day after tweeting that US Immigration and Customs Enforcement (ICE) would soon begin removing "millions of illegal aliens" from the country.
"We believe our country should be a sanctuary for law-abiding citizens, not for criminal aliens," Mr Trump told Florida supporters.
He also accused Democrats of seeking to legalise illegal immigration in order to boost their voting base, and said they "want to destroy our country as we know it".
On no issue are Democrats more extreme – and more depraved – than when it comes to Border Security. The Democrat Agenda of open borders is morally reprehensible. It is the great betrayal of the American Middle Class and our Country as a whole! #Trump2020
End of Twitter post by @realDonaldTrump
Mr Trump described his opponents as a "radical left-wing mob" who he said would bring socialism to the US.
"A vote for any Democrat in 2020 is a vote for the rise of radical socialism and the destruction of the American dream," he told the crowd.
He also praised the economy, criticised the Mueller investigation into alleged collusion between the 2016 Trump campaign and Russia, and referred to media covering the event as "fake news back there".
The president also elicited "lock her up" chants from supporters when he brought up Hillary Clinton, despite her not being in the 2020 race.
A 'trial run' for his pitch to America
On Tuesday night in Orlando, Florida, Donald Trump "officially" kicked off his presidential re-election bid.
Of course, everyone knew he was going to run for re-election. That was hardly a surprise. He filed his 2020 paperwork the day after his January 2017 inauguration, and he's been holding regular campaign-style rallies across key battleground states ever since.
And so on Tuesday in Florida - one of the biggest of electoral battlegrounds, a state the president absolutely must win in 2020 - it wasn't about whether Mr Trump was going to seek re-election. It was about how he's going to do it.
The event was an unveiling of sorts - a trial run of his pitch to the American people for four more years, before a raucous, red-hat-wearing crowd packing a 20,000-seat arena.
Read more from Anthony on Trump's Florida supporters
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Democratic contender Bernie Sanders responded to the campaign launch by tweeting that Mr Trump was "living in a parallel universe".
"Our most important job is to defeat the most dangerous president in the modern history of this country," he said.
What happened before his speech?
Thousands of supporters gathered outside of the venue ahead of the president's rally.
Some had camped outside the Amway Center arena in Orlando since early Monday morning.
Mr Trump himself had claimed online the numbers numbered "thousands" two days in advance of his appearance.
CBS News reports many of the voters in the queue listed the economy and immigration among their top concerns - issues Mr Trump is sure to address throughout his campaign.
Protesters also attended and after a successful GoFundMe campaign, the Baby Trump balloon that took to the skies above London during the president's recent UK visit also put in an appearance.
Social media videos from the event appeared to show members of a far-right group, the Proud Boys, being stopped by police trying to confront anti-Trump demonstrators.
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Who will take on Trump in 2020?
President Donald Trump is facing a wide field of Democratic contenders in 2020. So who has a shot at becoming the next president?
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